A common thought among many automotive enthusiasts and amateur racers is that independent rear suspension (IRS)-equipped cars are superior on the track than those with solid rear axles. These days though, many solid-axle cars keep up with or even outrun their IRS-equipped counterparts – but why? Simply put: a car with IRS will not necessarily outperform a well-designed solid-axle setup. This is especially true in cars with a significant front weight-bias like the Mustang.

A large part of the equation is how well a car shifts weight backward under acceleration. The weight needs to transfer to the rear tires in order to grip and put the power down (assuming a RWD configuration), but that transfer takes time to travel through the springs and shocks. On high-power cars, that delay results in traction loss. An exception to the rule is some modern mid- and rear-engine supercars, as the weight is already primarily over the rear wheels so they don’t need as much additional weight-shift to put down power.

The most effective way to speed up weight transfer is with suspension that fights the “squat” that happens under acceleration. This allows the weight to transfer without having to wait for the springs to compress and load up.

This is a problem for IRS-equipped cars, as setting up the suspension on these vehicles often results in unwanted roll steer characteristics, making it an undesirable solution.

Solid axles on the other hand, fare better. A torque arm design is good for putting power down and cornering as it allows for good anti-squat characteristics while keeping the rest of the geometry intact. Solid axles with torque arms are also more tolerant to ride-height changes than IRS cars.

If you do a lot of driving on bumpy surfaces, IRS comes into its own. It is usually better to reduce the amount of anti-squat in this situation, so the springs and shocks can work towards smoothing out the bumps in the road, which is accomplished by altering the angle of the rear lower control arms.

No matter whether your car is equipped with IRS or a solid rear axle, PBOC’s HPDE classes and track events will help you get the most out of it. Contact us today at (407) 804-0892 with any questions!